Size selectivity is one of the leading principles of nature. Cell membranes allow permeation of small molecules in the cell body while the large ones are excluded. A similar principle has been adopted in the membrane technology and it is widely used there. Size exclusion is also one of the often used methods in chromatographic separations. The first paper describing so called "gel filtration," involving the separation of proteins from salt, dates back to 1959. Further progress in size exclusion chromatography was made by Moore, J. C. J. Polym. Sci. A2, 842, 1964, who introduced macroporous poly(styrene-co-divinylbenzene) beads and developed gel permeation chromatography.
Porous polymer beads are generally produced by the co-polymerization of only a limited number of monomers and crosslinking agents. The broad spectrum of pore surface chemistries available for such beads is commonly obtained by chemical modification of the basic copolymers rather than by the co-polymerization of a monomer bearing the new group. (Sherrington et al., Syntheses and Separations Using Functional Polymers, Wiley, N.Y., 1989.) While in the former process the physical properties of the basic matrix remains unchanged and only its surface may be modified, in the later the functional monomers are partly buried inside the matrix and physical properties of the copolymers change when different polymerization feeds are used. Accordingly, the chemical modification of polymer beads is more frequent than is the direct copolymerization of functional monomers. For example, strong cation- and all anion-exchange resins are currently commercially produced by chemical modification of styrene-divinylbenzene copolymers, while only a weak cation-exchanger is produced by polymerization of a mixture containing acrylic acid.
The extent of modification of a porous polymer is typically controlled by the reaction kinetics, i.e., by concentration of reagent, reaction time and temperature, diffusion, neighboring group effects, etc. During such a modification, reaction of groups exposed in the easily available parts of the porous polymer is preferred. As the reaction proceeds, the groups located in less accessible parts react to a larger extent until all groups available are consumed. The kinetic control of the reaction path allows neither specification of locations that should be modified nor prevention of the reaction of some groups in defined regions of a porous bead. The method only controls the overall reaction conversion, i.e., the average content of modified groups, without defining the location thereof.
A far better approach, however, would be to develop a process to control not only the extent of modification but also the location of the groups undergoing reaction. It is an object of the present invention to do so by development of a porous material containing different size pores which pores can be selectively modified to have different surface properties.